1. Technical Field
This invention relates generally to the field of rapid prototyping (RP) manufacturing or processes.
2. Related Art
RP is a technology which has seen great advances since its initial application in the 1980""s. In one common embodiment known as stereolithography, RP manufacturing comprises a bath of curable liquid wherein some movable point within the bath is subjected to stimulation by a prescribed curing source. As the source is moved with respect to the bath or as the bath is moved with respect to the source, the point which undergoes solidification or curing is constantly made to move. The result is the construction of a solidified mass of cured material contained within the otherwise liquid bath. The region commonly solidified occurs at or very near the surface of the bath in most practical applications. As the liquid is solidified, the solid structure is progressively lowered into the bath allowing the uncured liquid to flow over the surface which is in turn subjected to the same process. By continuing to solidify these very thin layers or laminae, the solid object is built up into its final shape. Bonding of one layer to a previous layer is an inherent property of the process as is known in the art. An adequate description of this process can be found in U.S. Pat. No. 4,575,330 issued to Charles W. Hull, which is hereby incorporated by reference.
The main advantages of the RP process are its ability to drastically reduce the time between product conception and final design, and its ability to create complex shapes. More traditional modeling or prototyping is obtained from an iterative generation of a series of drawings which are analyzed by the design team, manufacturing, the consumer, and perhaps others until a tentative final design results which is considered viable. This agreed upon design is then created by casting and/or machining processes. If molds are needed, these must be fabricated as well. The finished prototype is then tested to determine whether it meets the criteria for which the part was designed. The design and review process is often tedious and tooling for the creation of the prototype is laborious and expensive. If the part is complex, then a number of interim components must first be assembled. The prototype itself is then constructed from the individual components.
Use of RP significantly reduces the expense and time needed between conception and completion of the prototype. Commonly, the concept is rendered in CAD (computer aided design). As this process is fully electronic, drawings are not required for fabrication. The CAD system is used to generate a compatible output data file that contains information on the part""s geometry. This file is typically converted into a xe2x80x9cslicedxe2x80x9d data file that contains information on the part""s cross-section at predetermined layer depths. The RP control system then regenerates each cross-section sequentially at the surface of the curable resin. The fabricated part can be analyzed by the team or used for various form, fit, and functional tests. Due to the rapid speed and low cost of the process, several designs can be fabricated and evaluated in a fraction of the time and for significantly less than it would take to machine each concept. Since the RP process creates the structure by the creation of very thin laminae, complex components with internal complexities can be easily rendered without requiring the assembly of a plurality of individual components.
A disadvantage of RP other than its initial cost for the technology is that the time associated with the creation of each part can be longer than desired. Since creation of the part occurs in a point-by-point, layer-by-layer process, the time necessary to produce a single part can become excessive. For instance, an arbitrary part of six cubic inches with a 50 percent fill ratio will require approximately six hours to image utilizing current stereolithographic techniques having a 0.005 inch layer build, a laser spot size of 0.010 inches, and a 100 inch per second laser rate assuming no losses. This estimate comprises imaging time alone and does not account for platform movement, sweeping of the resin surface, resin setting time, and mirror inertias that take considerable time between formation of each lamina. Reduction in fabrication times continue to be a desirable goal. Though the above description pertains to the process of stereolithography; the process, as well as the general advantages and disadvantages are similar for other RP technologies.
Another disadvantage of RP specific to stereolithography is that parts produced by this process leave the bath in a very soft state requiring a post-cure process. This too takes time, typically a minimum of 20 minutes. In this soft state, the part is very deformable. Since the part is removed from the bath in a fragile state, supports are often needed to assist in the part""s creation and to ensure proper post-curing without significant deformation. In fact, these supports are often vital to parts created by stereolithography especially those parts having overhanging or other unsupported features. The soft state associated with conventional stereolithography is inherently unavoidable for at least two reasons. The first is that the stereolithography though not as rapid as the present invention is still optimized for speed. Thus, increasing the exposure time of the laser at each point on the surface of the resin would significantly increase the processing time of the part. Secondly, to create the part, the laser is rastered without overlapping and the energy at the forming lamina has a gaussian distribution. This traps uncured photopolymer between cured lines. This could be avoided by overlapping the laser paths; however, this would also greatly increase processing time.
Accordingly, the development and production of a faster method to create prototypes and finished parts using RP technology is a desirable goal. Improvement to the existing processes would greatly increase the use of RP and would result in the continued advancement of technology in general due to the increased ease in the creation of complex parts.
Briefly, the method and apparatus comprises processing an entire cross-section of the object at one time. By reducing the time needed to stimulate the bath surface to form the laminae, the entire object could be formulated more quickly. Increasing the quantity of material stimulated at each time interval is a preferred way to perform this function. The invention therefore comprises a method of solidifying a discrete quantity of a curable medium by subjecting the surface of said medium to a prescribed energy source and controlling that source in such a fashion to cure only the portion desired while leaving the remainder of the medium uncured. To accurately image an entire cross section at any one time in this manner requires accurate control of the energy source. The means currently preferred to accomplish this is through use of reflective digital light switch technology. This technology was created by Texas Instruments (TI) and is currently referred to as deflectable beam spatial light modulation (SLM). TI refers to the process when applied to its typical applications under their common law trademark as Digital Light Processing (DLP). More specifically, they refer to the critical mechanism used as a Digital Micromirror Device (DMD). U.S. Pat. No. 5,061,049 for a xe2x80x9cSpatial light modulator and methodxe2x80x9d issued on Oct. 29, 1991 to L. Hornbeck of Texas Instruments provides the basic configuration of such a device. Further descriptions of this technology can be found in numerous white papers by TI as well as issued patents including among others, a presentation placed in writing originally given by Larry J. Hornbeck entitled xe2x80x9cDigital Light Processing for High-Brightness, High-Resolution Applicationsxe2x80x9d on Feb. 10-12, 1997 in San Jose Calif. A history of the development of the DMD can be found by the same author in an article titled xe2x80x9cFrom cathode rays to digital micromirrors: A history of electronic projection display technologyxe2x80x9d, TI Technical Journal, July-September 1998. In the alternative, a Thin-Film Micromirror Array (TMA), comprising a plurality of micromachined thin-film piezoelectric actuators, may also be used.
In simplified terms, a DMD is microelectromechanical device comprising a plurality of tiny mirrored surfaces which each can be independently pivoted from a first to a second position. The mirrors are formed into the surface of a semiconductor chip and through the application of an appropriate voltage to the circuitry built under each mirror, that mirror may be made to tilt to one side or another with respect to a plane normal to the semiconductor chip. Further, with respect to some fixed frame of reference, pivoting in one direction causes the mirror to reflect light whereas pivoting in the opposite direction causes the light to be deflected from the fixed frame of reference. As such, to a viewer within the frame of reference, the mirror is either fully on or fully off depending on the direction in which it is pivoted. Each of the mirrors can be independently controlled to be at either of the tilt angles. Since each mirror typically represents a single pixel, a black and white image can be generated by setting the appropriate mirrors to the appropriate position. Both color images and shades of gray are possible with this technology through the use of colored filters for the former and mirror modulation for the latter. The technology associated with color is not important to the present invention other than at an appropriate specific wavelength of the light spectrum best suited to cure an appropriate medium However, utilizing shades of gray by modulating individual mirrors does have some application to the present invention and will be more appropriately discussed below.
By coupling this spatial light modulation technology to rapid prototyping, great gains are possible in the speed of creation of the prototype. Similar to point by point prototyping, as each succeeding lamina is imaged, a quantity of uncured material is added and selectively cured to form an adjacent lamina. The laminae are adjoined one to the other so that the successive laminae form a continuous solid object comprising the cured material. Using spatial light modulation technology eliminates the need to scan the energy source over each point in turn prior to the solidification of an entire lamina.
In an embodiment, by way of example and not by way of limitation, the invention utilizes a gas discharge lamp as the radiation energy source. The invention is also adaptable to any radiant energy source capable of being reflected, provided the reflected energy satisfies the requirements of the reactive material. One such example includes lasers. The use of lasers in RP is known in the art since it is currently being utilized for selective laser sintering, stereolithography as described above, and laminated object manufacturing.
In accordance with the invention, a method of generating a three-dimensional object layer by layer from a medium, comprises the steps of providing at least one energy source, and at least one spatial light modulator array having a plurality of individually controlled elements which are each capable of selectively and at least bi-directionally reflecting energy from the energy source. The method further comprises receiving data which corresponds to a two-dimensional cross-section or individual lamina taken from the three-dimensional object desired to be formed and generating control signals to selectively direct a quantity of the individual reflecting elements in either a first or a second direction, one of the directions corresponding to that direction which reflects energy from the energy source off of the spatial light modulator array or arrays and onto the medium The energy reflected onto the medium causes that portion of the medium in receipt of the reflected energy to undergo a solidification reaction thereby forming a replica in the medium of the two-dimensional cross-section stored in the data. Once the cross-section is satisfactorily solidified, the cross-section is lowered into the medium, additional uncured material is added to the surface, and the process is repeated incrementally for each two-dimensional cross-section until the entire three-dimensional object is replicated in the medium.
One of the primary advantages of the present invention is the projected degree of cure of a part emerging from the resin bath Since the present invention cures an entire cross-section at the bath""s surface at a single time, parts do not contain trapped, uncured resin. Additionally any increase in exposure time, if required, would be minimal. The predicted imaging time for a single lamina created by the present invention of cross-sectional lithography is estimated to be on the order of one second versus 18 to 36 seconds for the same lamina utilizing conventional stereolithography. Increasing exposure by 20 percent would equate to adding roughly three minutes to the creation of a 1000 laminae part by cross-sectional lithography whereas it would add 1.2 hours to the creation of the same part utilizing conventional stereolithography. Since cross-sectional lithography cures an entire lamina in one increment, and since it does not require raster filling of the image, surface geometry is irrelevant with respect to time. As such one object of the present invention is to provide a method to substantially reduce the time necessary to generate a three-dimensional object utilizing an RP process.
A second embodiment of the present invention provides a microfabrication stereolithography system, comprising at least one reduction lens to reduce the size of the image reflected by the spatial light modulator, and an optical window which provides a focal point for the incoming energy source, as well as establishing and maintaining a planar curing surface.
A third embodiment of the present invention provides an inverted stereolithography system comprising an optical window located substantially near the bottom of the container.
The inverted system further includes a target platform mounted to the underside of the elevator platform, such that the part may be lifted away from the window as each layer is cured, and removed from the container after completion.
It is further advantage of the present invention to provide a method for imaging an entire cross-section of the object at each time interval by a process now known as cross-sectional lithography.
It is further advantage of the present invention to improve the speed of existing rapid prototyping technologies.
It is another advantage of the present invention to provide a method of reflecting an energy source in order to image an entire cross-sectional area of material simultaneously thereby creating a complete lamina of solidified material comprising one layer in a plurality of stacked laminae forming the solid object.
It is a further advantage of the present invention to provide a method adaptable for use in other RP processes which utilize beam sources conducive to reflection or transmission in the case of light valve technology such as electron and particle beam
Yet a further advantage of the present invention is to provide a method adaptable for use in RP processes which utilize any wavelength of radiation conducive to reflection from the electromagnetic spectrum.
Yet another advantage of the present invention is to improve the rapidity, economy, and desirability of rapid prototyping technology.
Another advantage is to provide a process which can create parts exhibiting improved final tolerances, reduction in material waste, reduction or elimination of the reliance on structural supports or posts, and a reduction or elimination of post-curing.
Still a further advantage of the present invention is to improve the accuracy of the process through the ability to precisely modulate the transmitted energy of the energy source.
Still further advantages are to provide the ability to create parts in any size with any accuracy through the use of a lens system. These and other advantages of the present invention will become more readily apparent from the detailed description given hereafter. However, it should be understood that a detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this description.